about summary refs log tree commit diff
path: root/absl/types/variant.h
blob: 55017ae194b07befe18c9af9f4408c38b9a84efd (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
// Copyright 2018 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// -----------------------------------------------------------------------------
// variant.h
// -----------------------------------------------------------------------------
//
// This header file defines an `absl::variant` type for holding a type-safe
// value of some prescribed set of types (noted as alternative types), and
// associated functions for managing variants.
//
// The `absl::variant` type is a form of type-safe union. An `absl::variant`
// should always hold a value of one of its alternative types (except in the
// "valueless by exception state" -- see below). A default-constructed
// `absl::variant` will hold the value of its first alternative type, provided
// it is default-constructable.
//
// In exceptional cases due to error, an `absl::variant` can hold no
// value (known as a "valueless by exception" state), though this is not the
// norm.
//
// As with `absl::optional`, an `absl::variant` -- when it holds a value --
// allocates a value of that type directly within the `variant` itself; it
// cannot hold a reference, array, or the type `void`; it can, however, hold a
// pointer to externally managed memory.
//
// `absl::variant` is a C++11 compatible version of the C++17 `std::variant`
// abstraction and is designed to be a drop-in replacement for code compliant
// with C++17.

#ifndef ABSL_TYPES_VARIANT_H_
#define ABSL_TYPES_VARIANT_H_

#include "absl/base/config.h"
#include "absl/utility/utility.h"

#ifdef ABSL_HAVE_STD_VARIANT

#include <variant>

namespace absl {
using std::bad_variant_access;
using std::get;
using std::get_if;
using std::holds_alternative;
using std::monostate;
using std::variant;
using std::variant_alternative;
using std::variant_alternative_t;
using std::variant_npos;
using std::variant_size;
using std::variant_size_v;
using std::visit;
}  // namespace absl

#else  // ABSL_HAVE_STD_VARIANT

#include <functional>
#include <new>
#include <type_traits>
#include <utility>

#include "absl/base/macros.h"
#include "absl/base/port.h"
#include "absl/meta/type_traits.h"
#include "absl/types/internal/variant.h"

namespace absl {

// -----------------------------------------------------------------------------
// absl::variant
// -----------------------------------------------------------------------------
//
// An 'absl::variant` type is a form of type-safe union. An `absl::variant` --
// except in exceptional cases -- always holds a value of one of its alternative
// types.
//
// Example:
//
//   // Construct a variant that holds either an integer or a std::string and
//   // assign it to a std::string.
//   absl::variant<int, std::string> v = std::string("abc");
//
//   // A default-contructed variant will hold a value-initialized value of
//   // the first alternative type.
//   auto a = absl::variant<int, std::string>();   // Holds an int of value '0'.
//
//   // variants are assignable.
//
//   // copy assignment
//   auto v1 = absl::variant<int, std::string>("abc");
//   auto v2 = absl::variant<int, std::string>(10);
//   v2 = v1;  // copy assign
//
//   // move assignment
//   auto v1 = absl::variant<int, std::string>("abc");
//   v1 = absl::variant<int, std::string>(10);
//
//   // assignment through type conversion
//   a = 128;         // variant contains int
//   a = "128";       // variant contains std::string
//
// An `absl::variant` holding a value of one of its alternative types `T` holds
// an allocation of `T` directly within the variant itself. An `absl::variant`
// is not allowed to allocate additional storage, such as dynamic memory, to
// allocate the contained value. The contained value shall be allocated in a
// region of the variant storage suitably aligned for all alternative types.
template <typename... Ts>
class variant;

// swap()
//
// Swaps two `absl::variant` values. This function is equivalent to `v.swap(w)`
// where `v` and `w` are `absl::variant` types.
//
// Note that this function requires all alternative types to be both swappable
// and move-constructible, because any two variants may refer to either the same
// type (in which case, they will be swapped) or to two different types (in
// which case the values will need to be moved).
//
template <typename... Ts>
void swap(variant<Ts...>& v, variant<Ts...>& w) noexcept(noexcept(v.swap(w))) {
  v.swap(w);
}

// variant_size
//
// Returns the number of alterative types available for a given `absl::variant`
// type as a compile-time constant expression. As this is a class template, it
// is not generally useful for accessing the number of alternative types of
// any given `absl::variant` instance.
//
// Example:
//
//   auto a = absl::variant<int, std::string>;
//   constexpr int num_types =
//       absl::variant_size<absl::variant<int, std::string>>();
//
//   // You can also use the member constant `value`.
//   constexpr int num_types =
//       absl::variant_size<absl::variant<int, std::string>>::value;
//
//   // `absl::variant_size` is more valuable for use in generic code:
//   template <typename Variant>
//   constexpr bool IsVariantMultivalue() {
//       return absl::variant_size<Variant>() > 1;
//   }
//
// Note that the set of cv-qualified specializations of `variant_size` are
// provided to ensure that those specializations compile (especially when passed
// within template logic).
template <class T>
struct variant_size;

template <class... Ts>
struct variant_size<variant<Ts...>>
    : std::integral_constant<std::size_t, sizeof...(Ts)> {};

// Specialization of `variant_size` for const qualified variants.
template <class T>
struct variant_size<const T> : variant_size<T>::type {};

// Specialization of `variant_size` for volatile qualified variants.
template <class T>
struct variant_size<volatile T> : variant_size<T>::type {};

// Specialization of `variant_size` for const volatile qualified variants.
template <class T>
struct variant_size<const volatile T> : variant_size<T>::type {};

// variant_alternative
//
// Returns the alternative type for a given `absl::variant` at the passed
// index value as a compile-time constant expression. As this is a class
// template resulting in a type, it is not useful for access of the run-time
// value of any given `absl::variant` variable.
//
// Example:
//
//   // The type of the 0th alternative is "int".
//   using alternative_type_0
//     = absl::variant_alternative<0, absl::variant<int, std::string>>::type;
//
//   static_assert(std::is_same<alternative_type_0, int>::value, "");
//
//   // `absl::variant_alternative` is more valuable for use in generic code:
//   template <typename Variant>
//   constexpr bool IsFirstElementTrivial() {
//       return std::is_trivial_v<variant_alternative<0, Variant>::type>;
//   }
//
// Note that the set of cv-qualified specializations of `variant_alternative`
// are provided to ensure that those specializations compile (especially when
// passed within template logic).
template <std::size_t I, class T>
struct variant_alternative;

template <std::size_t I, class... Types>
struct variant_alternative<I, variant<Types...>> {
  using type =
      variant_internal::VariantAlternativeSfinaeT<I, variant<Types...>>;
};

// Specialization of `variant_alternative` for const qualified variants.
template <std::size_t I, class T>
struct variant_alternative<I, const T> {
  using type = const typename variant_alternative<I, T>::type;
};

// Specialization of `variant_alternative` for volatile qualified variants.
template <std::size_t I, class T>
struct variant_alternative<I, volatile T> {
  using type = volatile typename variant_alternative<I, T>::type;
};

// Specialization of `variant_alternative` for const volatile qualified
// variants.
template <std::size_t I, class T>
struct variant_alternative<I, const volatile T> {
  using type = const volatile typename variant_alternative<I, T>::type;
};

// Template type alias for variant_alternative<I, T>::type.
//
// Example:
//
//   using alternative_type_0
//     = absl::variant_alternative_t<0, absl::variant<int, std::string>>;
//   static_assert(std::is_same<alternative_type_0, int>::value, "");
template <std::size_t I, class T>
using variant_alternative_t = typename variant_alternative<I, T>::type;

// holds_alternative()
//
// Checks whether the given variant currently holds a given alternative type,
// returning `true` if so.
//
// Example:
//
//   absl::variant<int, std::string> bar = 42;
//   if (absl::holds_alternative<int>(foo)) {
//       std::cout << "The variant holds an integer";
//   }
template <class T, class... Types>
constexpr bool holds_alternative(const variant<Types...>& v) noexcept {
  static_assert(
      variant_internal::UnambiguousIndexOfImpl<variant<Types...>, T,
                                               0>::value != sizeof...(Types),
      "The type T must occur exactly once in Types...");
  return v.index() ==
         variant_internal::UnambiguousIndexOf<variant<Types...>, T>::value;
}

// get()
//
// Returns a reference to the value currently within a given variant, using
// either a unique alternative type amongst the variant's set of alternative
// types, or the variant's index value. Attempting to get a variant's value
// using a type that is not unique within the variant's set of alternative types
// is a compile-time error. If the index of the alternative being specified is
// different from the index of the alternative that is currently stored, throws
// `absl::bad_variant_access`.
//
// Example:
//
//   auto a = absl::variant<int, std::string>;
//
//   // Get the value by type (if unique).
//   int i = absl::get<int>(a);
//
//   auto b = absl::variant<int, int>;
//
//   // Getting the value by a type that is not unique is ill-formed.
//   int j = absl::get<int>(b);     // Compile Error!
//
//   // Getting value by index not ambiguous and allowed.
//   int k = absl::get<1>(b);

// Overload for getting a variant's lvalue by type.
template <class T, class... Types>
constexpr T& get(variant<Types...>& v) {  // NOLINT
  return variant_internal::VariantCoreAccess::Access<
      variant_internal::IndexOf<T, Types...>::value>(v);
}

// Overload for getting a variant's rvalue by type.
// Note: `absl::move()` is required to allow use of constexpr in C++11.
template <class T, class... Types>
constexpr T&& get(variant<Types...>&& v) {
  return variant_internal::VariantCoreAccess::Access<
      variant_internal::IndexOf<T, Types...>::value>(absl::move(v));
}

// Overload for getting a variant's const lvalue by type.
template <class T, class... Types>
constexpr const T& get(const variant<Types...>& v) {
  return variant_internal::VariantCoreAccess::Access<
      variant_internal::IndexOf<T, Types...>::value>(v);
}

// Overload for getting a variant's const rvalue by type.
// Note: `absl::move()` is required to allow use of constexpr in C++11.
template <class T, class... Types>
constexpr const T&& get(const variant<Types...>&& v) {
  return variant_internal::VariantCoreAccess::Access<
      variant_internal::IndexOf<T, Types...>::value>(absl::move(v));
}

// Overload for getting a variant's lvalue by index.
template <std::size_t I, class... Types>
constexpr variant_alternative_t<I, variant<Types...>>& get(
    variant<Types...>& v) {  // NOLINT
  return variant_internal::VariantCoreAccess::Access<I>(v);
}

// Overload for getting a variant's rvalue by index.
// Note: `absl::move()` is required to allow use of constexpr in C++11.
template <std::size_t I, class... Types>
constexpr variant_alternative_t<I, variant<Types...>>&& get(
    variant<Types...>&& v) {
  return variant_internal::VariantCoreAccess::Access<I>(absl::move(v));
}

// Overload for getting a variant's const lvalue by index.
template <std::size_t I, class... Types>
constexpr const variant_alternative_t<I, variant<Types...>>& get(
    const variant<Types...>& v) {
  return variant_internal::VariantCoreAccess::Access<I>(v);
}

// Overload for getting a variant's const rvalue by index.
// Note: `absl::move()` is required to allow use of constexpr in C++11.
template <std::size_t I, class... Types>
constexpr const variant_alternative_t<I, variant<Types...>>&& get(
    const variant<Types...>&& v) {
  return variant_internal::VariantCoreAccess::Access<I>(absl::move(v));
}

// get_if()
//
// Returns a pointer to the value currently stored within a given variant, if
// present, using either a unique alternative type amongst the variant's set of
// alternative types, or the variant's index value. If such a value does not
// exist, returns `nullptr`.
//
// As with `get`, attempting to get a variant's value using a type that is not
// unique within the variant's set of alternative types is a compile-time error.

// Overload for getting a pointer to the value stored in the given variant by
// index.
template <std::size_t I, class... Types>
constexpr absl::add_pointer_t<variant_alternative_t<I, variant<Types...>>>
get_if(variant<Types...>* v) noexcept {
  return (v != nullptr && v->index() == I) ? std::addressof(absl::get<I>(*v))
                                           : nullptr;
}

// Overload for getting a pointer to the const value stored in the given
// variant by index.
template <std::size_t I, class... Types>
constexpr absl::add_pointer_t<const variant_alternative_t<I, variant<Types...>>>
get_if(const variant<Types...>* v) noexcept {
  return (v != nullptr && v->index() == I) ? std::addressof(absl::get<I>(*v))
                                           : nullptr;
}

// Overload for getting a pointer to the value stored in the given variant by
// type.
template <class T, class... Types>
constexpr absl::add_pointer_t<T> get_if(variant<Types...>* v) noexcept {
  return absl::get_if<variant_internal::IndexOf<T, Types...>::value>(v);
}

// Overload for getting a pointer to the const value stored in the given variant
// by type.
template <class T, class... Types>
constexpr absl::add_pointer_t<const T> get_if(
    const variant<Types...>* v) noexcept {
  return absl::get_if<variant_internal::IndexOf<T, Types...>::value>(v);
}

// visit()
//
// Calls a provided functor on a given set of variants. `absl::visit()` is
// commonly used to conditionally inspect the state of a given variant (or set
// of variants).
// Requires: The expression in the Effects: element shall be a valid expression
// of the same type and value category, for all combinations of alternative
// types of all variants. Otherwise, the program is ill-formed.
//
// Example:
//
//   // Define a visitor functor
//   struct GetVariant {
//       template<typename T>
//       void operator()(const T& i) const {
//         std::cout << "The variant's value is: " << i;
//       }
//   };
//
//   // Declare our variant, and call `absl::visit()` on it.
//   std::variant<int, std::string> foo = std::string("foo");
//   GetVariant visitor;
//   std::visit(visitor, foo);  // Prints `The variant's value is: foo'
template <typename Visitor, typename... Variants>
variant_internal::VisitResult<Visitor, Variants...> visit(Visitor&& vis,
                                                          Variants&&... vars) {
  return variant_internal::
      VisitIndices<variant_size<absl::decay_t<Variants> >::value...>::Run(
          variant_internal::PerformVisitation<Visitor, Variants...>{
              std::forward_as_tuple(absl::forward<Variants>(vars)...),
              absl::forward<Visitor>(vis)},
          vars.index()...);
}

// monostate
//
// The monostate class serves as a first alternative type for a variant for
// which the first variant type is otherwise not default-constructible.
struct monostate {};

// `absl::monostate` Relational Operators

constexpr bool operator<(monostate, monostate) noexcept { return false; }
constexpr bool operator>(monostate, monostate) noexcept { return false; }
constexpr bool operator<=(monostate, monostate) noexcept { return true; }
constexpr bool operator>=(monostate, monostate) noexcept { return true; }
constexpr bool operator==(monostate, monostate) noexcept { return true; }
constexpr bool operator!=(monostate, monostate) noexcept { return false; }


//------------------------------------------------------------------------------
// `absl::variant` Template Definition
//------------------------------------------------------------------------------
template <typename T0, typename... Tn>
class variant<T0, Tn...> : private variant_internal::VariantBase<T0, Tn...> {
  // Intentionally not qualifing `negation` with `absl::` to work around a bug
  // in MSVC 2015 with inline namespace and variadic template.
  static_assert(absl::conjunction<std::is_object<T0>, std::is_object<Tn>...,
                                  negation<std::is_array<T0> >,
                                  negation<std::is_array<Tn> >...,
                                  std::is_nothrow_destructible<T0>,
                                  std::is_nothrow_destructible<Tn>...>::value,
                "Attempted to instantiate a variant with an unsupported type.");

  friend struct variant_internal::VariantCoreAccess;

 private:
  using Base = variant_internal::VariantBase<T0, Tn...>;

 public:
  // Constructors

  // Constructs a variant holding a default-initialized value of the first
  // alternative type.
  constexpr variant() /*noexcept(see 111above)*/ = default;

  // Copy constructor, standard semantics
  variant(const variant& other) = default;

  // Move constructor, standard semantics
  variant(variant&& other) /*noexcept(see above)*/ = default;

  // Constructs a variant of an alternative type specified by overload
  // resolution of the provided forwarding arguments through
  // direct-initialization.
  //
  // Note: If the selected constructor is a constexpr constructor, this
  // constructor shall be a constexpr constructor.
  //
  // NOTE: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0608r1.html
  // has been voted passed the design phase in the C++ standard meeting in Mar
  // 2018. It will be implemented and integrated into `absl::variant`.
  template <
      class T,
      std::size_t I = std::enable_if<
          variant_internal::IsNeitherSelfNorInPlace<variant,
                                                    absl::decay_t<T>>::value,
          variant_internal::IndexOfConstructedType<variant, T>>::type::value,
      class Tj = absl::variant_alternative_t<I, variant>,
      absl::enable_if_t<std::is_constructible<Tj, T>::value>* =
          nullptr>
  constexpr variant(T&& t) noexcept(std::is_nothrow_constructible<Tj, T>::value)
      : Base(variant_internal::EmplaceTag<I>(), absl::forward<T>(t)) {}

  // Constructs a variant of an alternative type from the arguments through
  // direct-initialization.
  //
  // Note: If the selected constructor is a constexpr constructor, this
  // constructor shall be a constexpr constructor.
  template <class T, class... Args,
            typename std::enable_if<std::is_constructible<
                variant_internal::UnambiguousTypeOfT<variant, T>,
                Args...>::value>::type* = nullptr>
  constexpr explicit variant(in_place_type_t<T>, Args&&... args)
      : Base(variant_internal::EmplaceTag<
                 variant_internal::UnambiguousIndexOf<variant, T>::value>(),
             absl::forward<Args>(args)...) {}

  // Constructs a variant of an alternative type from an initializer list
  // and other arguments through direct-initialization.
  //
  // Note: If the selected constructor is a constexpr constructor, this
  // constructor shall be a constexpr constructor.
  template <class T, class U, class... Args,
            typename std::enable_if<std::is_constructible<
                variant_internal::UnambiguousTypeOfT<variant, T>,
                std::initializer_list<U>&, Args...>::value>::type* = nullptr>
  constexpr explicit variant(in_place_type_t<T>, std::initializer_list<U> il,
                             Args&&... args)
      : Base(variant_internal::EmplaceTag<
                 variant_internal::UnambiguousIndexOf<variant, T>::value>(),
             il, absl::forward<Args>(args)...) {}

  // Constructs a variant of an alternative type from a provided index,
  // through value-initialization using the provided forwarded arguments.
  template <std::size_t I, class... Args,
            typename std::enable_if<std::is_constructible<
                variant_internal::VariantAlternativeSfinaeT<I, variant>,
                Args...>::value>::type* = nullptr>
  constexpr explicit variant(in_place_index_t<I>, Args&&... args)
      : Base(variant_internal::EmplaceTag<I>(), absl::forward<Args>(args)...) {}

  // Constructs a variant of an alternative type from a provided index,
  // through value-initialization of an initializer list and the provided
  // forwarded arguments.
  template <std::size_t I, class U, class... Args,
            typename std::enable_if<std::is_constructible<
                variant_internal::VariantAlternativeSfinaeT<I, variant>,
                std::initializer_list<U>&, Args...>::value>::type* = nullptr>
  constexpr explicit variant(in_place_index_t<I>, std::initializer_list<U> il,
                             Args&&... args)
      : Base(variant_internal::EmplaceTag<I>(), il,
             absl::forward<Args>(args)...) {}

  // Destructors

  // Destroys the variant's currently contained value, provided that
  // `absl::valueless_by_exception()` is false.
  ~variant() = default;

  // Assignment Operators

  // Copy assignement operator
  variant& operator=(const variant& other) = default;

  // Move assignment operator
  variant& operator=(variant&& other) /*noexcept(see above)*/ = default;

  // Converting assignment operator
  //
  // NOTE: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0608r1.html
  // has been voted passed the design phase in the C++ standard meeting in Mar
  // 2018. It will be implemented and integrated into `absl::variant`.
  template <
      class T,
      std::size_t I = std::enable_if<
          !std::is_same<absl::decay_t<T>, variant>::value,
          variant_internal::IndexOfConstructedType<variant, T>>::type::value,
      class Tj = absl::variant_alternative_t<I, variant>,
      typename std::enable_if<std::is_assignable<Tj&, T>::value &&
                              std::is_constructible<Tj, T>::value>::type* =
          nullptr>
  variant& operator=(T&& t) noexcept(
      std::is_nothrow_assignable<Tj&, T>::value&&
          std::is_nothrow_constructible<Tj, T>::value) {
    variant_internal::VisitIndices<sizeof...(Tn) + 1>::Run(
        variant_internal::VariantCoreAccess::MakeConversionAssignVisitor(
            this, absl::forward<T>(t)),
        index());

    return *this;
  }


  // emplace() Functions

  // Constructs a value of the given alternative type T within the variant.
  //
  // Example:
  //
  //   absl::variant<std::vector<int>, int, std::string> v;
  //   v.emplace<int>(99);
  //   v.emplace<std::string>("abc");
  template <
      class T, class... Args,
      typename std::enable_if<std::is_constructible<
          absl::variant_alternative_t<
              variant_internal::UnambiguousIndexOf<variant, T>::value, variant>,
          Args...>::value>::type* = nullptr>
  T& emplace(Args&&... args) {
    return variant_internal::VariantCoreAccess::Replace<
        variant_internal::UnambiguousIndexOf<variant, T>::value>(
        this, absl::forward<Args>(args)...);
  }

  // Constructs a value of the given alternative type T within the variant using
  // an initializer list.
  //
  // Example:
  //
  //   absl::variant<std::vector<int>, int, std::string> v;
  //   v.emplace<std::vector<int>>({0, 1, 2});
  template <
      class T, class U, class... Args,
      typename std::enable_if<std::is_constructible<
          absl::variant_alternative_t<
              variant_internal::UnambiguousIndexOf<variant, T>::value, variant>,
          std::initializer_list<U>&, Args...>::value>::type* = nullptr>
  T& emplace(std::initializer_list<U> il, Args&&... args) {
    return variant_internal::VariantCoreAccess::Replace<
        variant_internal::UnambiguousIndexOf<variant, T>::value>(
        this, il, absl::forward<Args>(args)...);
  }

  // Destroys the current value of the variant (provided that
  // `absl::valueless_by_exception()` is false, and constructs a new value at
  // the given index.
  //
  // Example:
  //
  //   absl::variant<std::vector<int>, int, int> v;
  //   v.emplace<1>(99);
  //   v.emplace<2>(98);
  //   v.emplace<int>(99);  // Won't compile. 'int' isn't a unique type.
  template <std::size_t I, class... Args,
            typename std::enable_if<
                std::is_constructible<absl::variant_alternative_t<I, variant>,
                                      Args...>::value>::type* = nullptr>
  absl::variant_alternative_t<I, variant>& emplace(Args&&... args) {
    return variant_internal::VariantCoreAccess::Replace<I>(
        this, absl::forward<Args>(args)...);
  }

  // Destroys the current value of the variant (provided that
  // `absl::valueless_by_exception()` is false, and constructs a new value at
  // the given index using an initializer list and the provided arguments.
  //
  // Example:
  //
  //   absl::variant<std::vector<int>, int, int> v;
  //   v.emplace<0>({0, 1, 2});
  template <std::size_t I, class U, class... Args,
            typename std::enable_if<std::is_constructible<
                absl::variant_alternative_t<I, variant>,
                std::initializer_list<U>&, Args...>::value>::type* = nullptr>
  absl::variant_alternative_t<I, variant>& emplace(std::initializer_list<U> il,
                                                   Args&&... args) {
    return variant_internal::VariantCoreAccess::Replace<I>(
        this, il, absl::forward<Args>(args)...);
  }

  // variant::valueless_by_exception()
  //
  // Returns false if and only if the variant currently holds a valid value.
  constexpr bool valueless_by_exception() const noexcept {
    return this->index_ == absl::variant_npos;
  }

  // variant::index()
  //
  // Returns the index value of the variant's currently selected alternative
  // type.
  constexpr std::size_t index() const noexcept { return this->index_; }

  // variant::swap()
  //
  // Swaps the values of two variant objects.
  //
  // TODO(calabrese)
  //   `variant::swap()` and `swap()` rely on `std::is_(nothrow)_swappable()`
  //   which is introduced in C++17. So we assume `is_swappable()` is always
  //   true and `is_nothrow_swappable()` is same as `std::is_trivial()`.
  void swap(variant& rhs) noexcept(
      absl::conjunction<std::is_trivial<T0>, std::is_trivial<Tn>...>::value) {
    return variant_internal::VisitIndices<sizeof...(Tn) + 1>::Run(
        variant_internal::Swap<T0, Tn...>{this, &rhs}, rhs.index());
  }
};

// We need a valid declaration of variant<> for SFINAE and overload resolution
// to work properly above, but we don't need a full declaration since this type
// will never be constructed. This declaration, though incomplete, suffices.
template <>
class variant<>;

//------------------------------------------------------------------------------
// Relational Operators
//------------------------------------------------------------------------------
//
// If neither operand is in the `variant::valueless_by_exception` state:
//
//   * If the index of both variants is the same, the relational operator
//     returns the result of the corresponding relational operator for the
//     corresponding alternative type.
//   * If the index of both variants is not the same, the relational operator
//     returns the result of that operation applied to the value of the left
//     operand's index and the value of the right operand's index.
//   * If at least one operand is in the valueless_by_exception state:
//     - A variant in the valueless_by_exception state is only considered equal
//       to another variant in the valueless_by_exception state.
//     - If exactly one operand is in the valueless_by_exception state, the
//       variant in the valueless_by_exception state is less than the variant
//       that is not in the valueless_by_exception state.
//
// Note: The value 1 is added to each index in the relational comparisons such
// that the index corresponding to the valueless_by_exception state wraps around
// to 0 (the lowest value for the index type), and the remaining indices stay in
// the same relative order.

// Equal-to operator
template <typename... Types>
constexpr variant_internal::RequireAllHaveEqualT<Types...> operator==(
    const variant<Types...>& a, const variant<Types...>& b) {
  return (a.index() == b.index()) &&
         variant_internal::VisitIndices<sizeof...(Types)>::Run(
             variant_internal::EqualsOp<Types...>{&a, &b}, a.index());
}

// Not equal operator
template <typename... Types>
constexpr variant_internal::RequireAllHaveNotEqualT<Types...> operator!=(
    const variant<Types...>& a, const variant<Types...>& b) {
  return (a.index() != b.index()) ||
         variant_internal::VisitIndices<sizeof...(Types)>::Run(
             variant_internal::NotEqualsOp<Types...>{&a, &b}, a.index());
}

// Less-than operator
template <typename... Types>
constexpr variant_internal::RequireAllHaveLessThanT<Types...> operator<(
    const variant<Types...>& a, const variant<Types...>& b) {
  return (a.index() != b.index())
             ? (a.index() + 1) < (b.index() + 1)
             : variant_internal::VisitIndices<sizeof...(Types)>::Run(
                   variant_internal::LessThanOp<Types...>{&a, &b}, a.index());
}

// Greater-than operator
template <typename... Types>
constexpr variant_internal::RequireAllHaveGreaterThanT<Types...> operator>(
    const variant<Types...>& a, const variant<Types...>& b) {
  return (a.index() != b.index())
             ? (a.index() + 1) > (b.index() + 1)
             : variant_internal::VisitIndices<sizeof...(Types)>::Run(
                   variant_internal::GreaterThanOp<Types...>{&a, &b},
                   a.index());
}

// Less-than or equal-to operator
template <typename... Types>
constexpr variant_internal::RequireAllHaveLessThanOrEqualT<Types...> operator<=(
    const variant<Types...>& a, const variant<Types...>& b) {
  return (a.index() != b.index())
             ? (a.index() + 1) < (b.index() + 1)
             : variant_internal::VisitIndices<sizeof...(Types)>::Run(
                   variant_internal::LessThanOrEqualsOp<Types...>{&a, &b},
                   a.index());
}

// Greater-than or equal-to operator
template <typename... Types>
constexpr variant_internal::RequireAllHaveGreaterThanOrEqualT<Types...>
operator>=(const variant<Types...>& a, const variant<Types...>& b) {
  return (a.index() != b.index())
             ? (a.index() + 1) > (b.index() + 1)
             : variant_internal::VisitIndices<sizeof...(Types)>::Run(
                   variant_internal::GreaterThanOrEqualsOp<Types...>{&a, &b},
                   a.index());
}

}  // namespace absl

namespace std {

// hash()
template <>  // NOLINT
struct hash<absl::monostate> {
  std::size_t operator()(absl::monostate) const { return 0; }
};

template <class... T>  // NOLINT
struct hash<absl::variant<T...>>
    : absl::variant_internal::VariantHashBase<absl::variant<T...>, void,
                                              absl::remove_const_t<T>...> {};

}  // namespace std

#endif  // ABSL_HAVE_STD_VARIANT

namespace absl {
namespace variant_internal {

// Helper visitor for converting a variant<Ts...>` into another type (mostly
// variant) that can be constructed from any type.
template <typename To>
struct ConversionVisitor {
  template <typename T>
  To operator()(T&& v) const {
    return To(std::forward<T>(v));
  }
};

}  // namespace variant_internal

// ConvertVariantTo()
//
// Helper functions to convert an `absl::variant` to a variant of another set of
// types, provided that the alternative type of the new variant type can be
// converted from any type in the source variant.
//
// Example:
//
//   absl::variant<name1, name2, float> InternalReq(const Req&);
//
//   // name1 and name2 are convertible to name
//   absl::variant<name, float> ExternalReq(const Req& req) {
//     return absl::ConvertVariantTo<absl::variant<name, float>>(
//              InternalReq(req));
//   }
template <typename To, typename Variant>
To ConvertVariantTo(Variant&& variant) {
  return absl::visit(variant_internal::ConversionVisitor<To>{},
                     std::forward<Variant>(variant));
}

}  // namespace absl

#endif  // ABSL_TYPES_VARIANT_H_